Modem At Commands; Modes Of Operation; Command Structure - Multitech ISIHI-2S User Manual

Chapter 5 - AT Commands

Modem AT Commands

ISIHP modems are controlled by instructions called AT commands, so called because the attention characters,
AT, precede each command or sequence of commands (known as a command string). You can send commands to
the modem from your keyboard while in terminal mode, or you can use communications software to issue these
commands automatically.
The modem is in command mode when it is not dialing or online. When it is in command mode, you have access
to a complete communications system that allows you to use several features, including the basic AT command
set described in this chapter. Using the basic AT command set, you can enter phone numbers for automatic
dialing, configure modem options, and monitor telephone activity. In addition, you can command your modem to
perform advanced features such as error correction, data compression, speed conversion, and more.
This chapter describes the modem's operational modes and shows you how to use each modem AT commands.
These commands and responses are compatible with all systems and with all data communications software
using the AT command set.

Modes of Operation

The modem operates in two basic functional modes: command mode and online mode. (There is also an in-
between state, wait-for-carrier, in which the modem is out of command mode but not yet online.) When you turn
on the modem, it is in command mode and is ready to accept and respond to commands from your keyboard or
software.
The modem enters online mode after it dials, connects with another modem, and detects a valid carrier signal. If
it does not detect a carrier signal within the time frame controlled by the S-register S7, the modem abandons the
call and reenters command mode.
You can make the modem enter online mode without dialing by entering AT and then D (dial) or A (force answer
mode). The modem exits online mode if the carrier signal is lost or intentionally dropped. When this happens,
the modem hangs up and reenters command mode. By sending certain escape characters to the modem while
online, you can make it enter command mode without losing the carrier signal. While waiting to establish the
carrier, you can type any character from the keyboard to make the modem to go back to the command mode.

Command Structure

You can control a wide variety of modem operations and options when the modem is in command mode. AT
commands tell the modem to dial a number, to answer a call, to operate at a certain speed, to use a certain
compression technique, and many other functions. AT commands consist of one or two letters, which may be
preceded by an ampersand (&), a percent character (%), or a slash character (/). The Q command, for example,
determines whether the modem returns result codes, while the &Q command selects the asynchronous
communications mode.
A parameter after a command (0, 1, 2, etc.) tells the modem which option to use. If you do not specify a
parameter, the modem assumes the 0 (zero) option. E, for example, is the same as E0. You can issue several
commands on a single line (a command string) as long as the line does not exceed 40 characters.
Note: Each character in a command counts toward the 40 character command line maximum. Example: Q1 is
a single command, but it counts as two characters in the command line.
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